My best guess here is that the engine will have been tilted for towing? In which case all conventional carb float valve logic goes out the window, as gravity also plays a significant part i nthe ioperatio nof that valve. Coupled with the ambient level in the carb being at 45 degrees or so to "normal".
Fuel takns if the breather is shut and properly sealed (I guess you donlt have Hulk tanks?
) then the expansion of the air with the heat of the sun as you tow could be enough to auto pump. The only thing stopping it is the float valve which as above is about as much use a sa chocolate teapot at that angle.
Re. the explosion, depending how much had been pumped into the cowl and depending on where it pooled / ran when the engine was moved to "operating angle" there is a starter with a sparky set of brushes, and could also have been a backfire.....
I would agree with checking your electrics, but also look for scorch marks roud the inlet passages too.....
I guess you may have an older version of mine. There is no air filter per se, but should probably be an air box?
If it's a "Brown stripe" livery on the cowl, does the top of the cowl attach to an open square box? that could be an air filter?
You got a pic? if it's the vintage I think it is the air intake body holds the cowl on! - Mr Boatjumbles in Bellshill (google will find his number) has one breaking for spares at the moment....... I'm going to collect a bit for a decrepit Johnson tonight or tomorrow, I can ask if he still has the air box if you want.